<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 150px;
            height: 20px;
            border: 1px solid red;
            border-radius: 10px;
            
        }
        .son{
            width: 50%;
            height: 20px;
            background-color: red;
            border-radius: 10px;
            transition: all ease .7s;
        }
        .box:hover .son{
            width: 100%;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="son">

        </div>
    </div>
</body>
</html>